Many signaling and trafficking proteins act as GTPases: proteins whose activity depends on whether they bind guanosine diphosphate (GDP) or guanosine triphosphate (GTP).

The common switching cycle is

GDP-bound state
   ↓ GDP released, GTP binds
GTP-bound state
   ↓ GTP hydrolysis
GDP-bound state

For many GTPases, the GTP-bound conformation binds downstream partners more strongly and is described as active, while the GDP-bound form is less active. This is a common pattern, not a statement that GTP itself is a universal activating molecule.

Nucleotide state controls protein conformation

GTP and GDP differ by one phosphate group. Binding either nucleotide stabilizes somewhat different protein conformations, and those structural differences change which partner proteins can bind.

Thus the switch works by state-dependent molecular recognition.

Two regulator classes control switching speed

A guanine-nucleotide exchange factor (GEF) promotes release of GDP. Because cellular GTP is available, GTP can then bind, driving the protein toward its active state.

A GTPase-activating protein (GAP) accelerates hydrolysis of bound GTP to GDP and inorganic phosphate, driving the switch toward its inactive state.

The logic is

GEF favors GDP release → GTP-bound state
GAP accelerates GTP hydrolysis → GDP-bound state

The switch consumes GTP to reset directionally

Hydrolysis

$$\mathrm{GTP+H_2O\rightarrow GDP+P_i}$$

makes the cycle chemically directional. The GTPase does not simply oscillate at equilibrium between two equally persistent states.

Different GTPase families use the same core logic in different cellular tasks. Heterotrimeric G proteins transmit signals from some membrane receptors; Ras-family proteins relay growth signals; other small GTPases regulate membrane trafficking and cytoskeletal organization.

The reusable concept is therefore a protein whose interaction state is coupled to a GDP/GTP cycle controlled by exchange and hydrolysis.